Cysteine is nutritionally important for protein synthesis, glutathione formation, and as a precursor of sulfate, taurine, thiosulfate and other metabolites. Various metabolites of cysteine are involved in the detoxification of drugs and xenobiotics or function as protecting agents against radiation damage. The long-term goals of this project are (1) to clarify the roles of various metabolic pathways of cysteine metabolism in mammalian tissues and in the intact animal and (2) to determine how flux through these metabolic pathways is altered during the life cycle (pre- and postnatal development, aging, pregnancy and lactation) and in response to dietary changes. Specific aims are (1) to develop methods for the separation and measurement of low levels of sulfur anions in physiological samples; (2) to determine the major pathways for the intermediary metabolism of cysteine sulfur in the liver; (3) to determine the major pathways for the intermediary metabolism of cysteine in nonhepatic tissues; (4) to determine the role of cysteamine in cysteine catabolism and to determine the products of cysteamine metabolism; and (5) to determine some of the effects of diet, age, sex and physiological changes on cysteine metabolism in mammalian tissues. Studies will be carried out using freshly isolated hepatocytes, kidney tubules, enterocytes or myocytes from rats or cats. Different tissues will be studied because it is likely that cysteine metabolism is substantially different in liver, kidney, intestine and heart, and nonhepatic tissues may play a role in cysteine metabolism in the intact animal. Rats and cats will be used because these two species metabolize cysteine quite differently. The rat has a high capacity for taurine synthesis, whereas the cat, like man, synthesizes very little taurine from cysteine. Previous work in our lab has indicated that the cysteinesulfinate pathway, currently accepted by many investigators as the major pathway of cysteine metabolism, plays only a minor role in the overall oxidation of cysteine. A continued systematic effort to elucidate the details of cysteine catabolism in mammalian tissues is needed.